1. Technical Field
The present disclosure relates to antenna design and more specifically to a solderless antenna feed for transmitting and/or receiving a circularly polarized microwave signal.
2. Introduction
Microwave transmission signals are typically transmitted with either linear or circular polarization. In linear polarization, the receiving antenna aligns its frame of reference with the transmitting antenna to achieve acceptable communications. Because this frame of reference can change depending on factors such as feed and reflector orientation, as well as Faraday rotation, antenna designers often exchange linear polarization for circular polarization, which is less affected by such factors. Circular polarization creates a rotating electric field resulting from two orthogonal linear components Ex and Ey, where both Ex and Ey have sinusoidally varying magnitudes equal in amplitude and 90° out of phase with one another. As Ex and Ey vary sinusoidally, a rotating signal is created by combining the horizontal polarization Ex and the vertical polarization Ey.
A common method of creating a circularly polarized signal is to connect a single antenna patch or a set of antenna patches to a feed network which rotates sequentially, with uniform angular spacing between feed points. Due to uniform angular spacing, uniform phase differences of 90° exist between each feed point. As the feed network rotates the signal phase, the feed points sequentially contact the antenna patch or patches with the signal 90° out of phase at each contacting feed point, creating a rotating signal within the antenna.
Microstrip transmission lines, also referred to as circuits, are commonly used to create accurate feed networks. Microstrip transmission lines are pieces of conductive material in the form of narrow strips near a wider grounded conductor which conduct the signal in this application to antenna feed points such that the signal undergoes a 90° phase shift between each feed point. This phase shift occurs by determining specific lengths of the transmission lines to provide the appropriate phase shifts. Because of the thin, planar nature of microstrip transmission lines, a dielectric substrate, which could be solid or a lightweight rigid honeycomb material, often supports the microstrip transmission lines. One disadvantage of traditional microstrip transmission line designs is the use of solder, and particularly the need to solder the microstrip transmission lines to the input points and antenna feed points. Extreme heat, cold, corrosion, or vibration, such as those found in space-based applications, can damage solder joints and break or reduce the signal transmission and reception characteristics of the antenna.
In an ideal, perfect antenna, the creation of the circularly polarized microwave signal would have no energy loss. However for real antennas, energy is lost in three ways: A small fraction of the energy is dissipated as heat and is minimized by using good conducting materials. More significant amounts of energy are lost in the feed network by being reflected back and when the signal conversion to transmitted radiation or from received radiation involves cross-polarization. The reflection loss is minimized by optimizing wave impedance matching in the feed network. Circular cross-polarization occurs due to the lack of a perfect 90° phase shift and/or equal signal amplitude between the two orthogonal linear field components. A cross-polarized signal is a signal polarized orthogonally to the desired polarization. For example, if a transmitting, circularly polarized antenna is creating a Right Hand Circularly Polarized (RHCP) signal, a cross-polarization signal can also be created which is Left Hand Circularly Polarized (LHCP). The cross-polarization weakens the effective signal strength of the intended signal. Therefore, reduction of cross-polarization transmitted or received by the antenna is a desirable characteristic.